In the power saving mode of the current institute of electrical and electronics engineers (IEEE) 802.11 standard, a terminal alternates between doze and awake states: the doze state being unable to transmit/receive and the awake state being able to transmit/receive with full power supply. The awake interval of the terminal is identical with a Beacon Interval of an access point (AP).
The AP buffers downlink data to the terminals in the doze state and notifies the terminals of the presence of the data for them, when they are awaken, using a Partial Virtual Bitmap field of a traffic indication map (TIM) in the beacon. The AP allocates an association ID (AID) to every terminal operating in the power saving mode. Each AID corresponds to each bit of the Partial Virtual Bitmap field. In this way, the AP notifies the terminals of the presence of the buffered data for them with the Partial Virtual Bitmap field indicating the AIDs of the terminals having buffered data present for them.
The terminals having the buffered data transmit a power save-poll (PS-Poll) frame according to the distributed coordination function (DCF) of IEEE 802.11 standard as in a contention-based access scheme. The AP transmits the buffered downlink data to the terminals which have transmitted the PS-Poll frames.
The contention-based DCF has a drawback in that the probability in which the terminals transmit the PS-Poll simultaneously increases in proportion to the number of contending terminals. This indicates that the PS-Poll transmission failure probability increases as the number of terminals having downlink data to receive increases in the contention-based DCF.
Furthermore, since the terminals transmitting the PS-Poll are determined based on randomness, it is difficult to guarantee PS-Poll transmission fairness for all competing terminals in the contention-based DCF.
There are some methods of the related art proposed to address the above issues. Among them, the Proxy carrier sense multiple access (CSMA) is a technique in which the AP selects some of the AIDs of the terminal having downlink data buffered and indicates them in the Partial Virtual Bitmap field so as to avoid excessive contention. Station Grouping is a technique of assigning priorities to the terminals to create a group of terminals to be indicated in the Partial Virtual Bitmap field for fair PS-Poll transmission in the course of performing the Proxy CSMA. TIM monitoring is a technique in which a terminal determines the number of terminals to contend by referencing a TIM so as to select a backoff value for use in the area corresponding to the number of the terminals.
The above-described techniques of the related art are disadvantageous in terms of low effectiveness in comparison to their high implementation complexities. For example, although it is possible to reduce the PS-Poll collision probability to some extent, the techniques of the related art cannot avoid PS-Poll collision completely. Furthermore, in the respect that the Proxy CSMA and Station Grouping techniques are implemented in the AP while the TIM Monitoring technique is in the terminal, these techniques of the related art are distributive so as to make it difficult to implement integrally. Although the techniques of the related art make it possible to expect partial collision avoidance effect with modification of the operations of the terminal and AP, there is a need of an enhanced PS-Poll transmission method capable of avoiding PS-Poll collision efficiently only with the modification of the operation of the terminal.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.